Mollusc invaders in the Thames – a mark of the Anthropocene

In the last few decades, the life of London’s River Thames has been transformed. The kinds of river-shells that have lived in the Thames while generations of Londoners built up the city – and have lived there since the end of the last Ice Age – are now rare, outcompeted and replaced by a massive influx of invasive species.

A new study led by Stephen Himson, Jan Zalasiewicz and Mark Williams at our School of Geography, Geology and the Environment, with colleagues including Dr David Aldridge of the University of Cambridge, has demonstrated the speed and scale of the change, and shown how it links with the accelerating global changes to the Earth’s biodiversity.

Sampling of the river sediments, made possible by engineering works at Teddington Lock, showed the extraordinary scale of the takeover. Over 95% of all of the river shells were of two invasive species, the zebra mussel and the Asian clam. The zebra mussel, originally from eastern Europe, has spread through western Europe to the UK since the 19th century. The Asian clam was only discovered in the UK in 2004, and is already the dominant mollusc sampled in the Thames below Teddington. It is a remarkable takeover.

Stephen Himson, Postgraduate Researcher at the University of Leicester said: “When we arrived and walked onto the river bed, the scale of the takeover was immediately apparent. Thick mats of shells covered the channel on both sides of the river, composed almost entirely of invasive organisms. We found shells of 34 individuals native to the Thames the whole day we were working there. To put that in context, we were observing over 7,000 invasive molluscs per square meter.

“Considering the Asian clam only arrived in the River Thames 15 years ago, it has, in combination with the zebra mussel, decimated the local mollusc population and is a stark reminder of the unintended consequences global shipping can have on native ecosystems.”

The Anthropocene is the proposed new epoch in which human activities dominate the surface geology of Earth. The researchers tracked the spread of the zebra mussel and the Asian clam around the world, to show how in many areas they might become time-markers for the Anthropocene. As future fossils that would suddenly crowd the strata at this level, they would be one of the clear signals to far-future palaeontologists, that the Earth’s biology was being fundamentally transformed.